It is of interest to CBER whether cell lines used for product production contain virus or virus-like particles or viral nucleic acid sequences or support the growth of viruses infectious for humans. The Chinese Hamster Ovary (CHO) cell line is used as the cell substrate for the recently licensed tissue plasminogen activator Activase, and additional recombinant products made in CHO cells are currently in clinical trials under IND. CHO cells contain endogenous retroviral-like particles visible by electron microscopy. These particles are not infectious and do not demonstrate reverse transcriptase activity, and are thought to be similar to the intracisternal A particles commonly found in mouse cell substrates used for monoclonal antibody production. An electron microscopic study was therefore undertaken to determine whether growth factors or cytokines (which might be manufactured in CHO cells) might affect these endogenous particles. The results indicate increases in particles from 30-100% after treatment of CHO cells with several cytokines (GM-CSF, IFN, TNF) or with the phorbol ester TPA. A manuscript has recently been published in In Vitro Toxicology (1991). We are currently examining whether Sf9 cells (an insect cell line used to produce recombinant products in the Baculovirus expression system) can be infected by arboviruses. Twelve arboviruses have been selected and are currently being tested for their ability to infect or be maintained in Sf9 cells; to date two of those tested apparently can infect SF9 cells and be transferred to Vero cells by cocultivation or via filtered supernatants. In addition to plaque titration, cultures are being examined by electron microscopy.